1. Field of the Invention
The present invention relates to an optical amplifier for use in optical communications equipment.
2. Description of the Related Art
In recent years, wavelength division multiplexing (WDM) transmission systems have been introduced. In transmitting stations, increasing the number of signal channels results in an increase in the number of post amplifiers used. An optical amplifier takes up a lot of space in optical transmitting equipment. It is therefore desired that the optical amplifier be decreased in size and produced in integrated form.
In the optical amplifier, a laser diode (LD) is used as a pumping light source and a component that gives off heat generated by the LD is associated with the light source. Of components that construct the optical amplifier, the light source and its associated radiator are very large, preventing the optical amplifier from becoming reduced in size. Thus, separating the pumping light source from the optical amplifier and grouping components necessary for use as a pumping light source, such as a light source and its associated driving circuit, as a pumping light source unit were thought of.
FIGS. 1A, 1B and 1C show arrangements of conventional pumping light source units.
As shown in FIGS. 1A, 1B and 1C, three types of pumping light source units may be considered (note that the example of FIG. 1A uses four pumping LDs). The example of FIG. 1C is used in the WDM system developed by CIENA (see catalog xe2x80x9cCIENA Multiwave Line Amplifier Block Diagramxe2x80x9d).
In FIGS. 1A to 1C, 1 denotes a circuit for driving a pumping light source, 2, 3, 4, 5 and 8; pumping light sources, 6; a polarization beam splitter (PBS), 7; a wavelength division multiplexing (WDM) coupler, 9; an optical splitter for separating pumping light, and 10; an optical coupler/splitter for coupling and separating pumping light.
FIG. 1A, each of the four pumping light sources 2, 3, 4 and 5 is driven (for example, supplied with current) by a respective one of the four pumping light driving circuits 1 to produce light. The light from each of the pumping light sources 2 to 5 is output as linearly polarized light. The beams of light output from the pumping light sources 2 and 3 can be set substantially equal to each other in wavelength but will be differently polarized, The beams of light which are substantially equal to each other in wavelength but differently polarized are polarization-coupled by the polarization beam splitter 6. The pumping light sources 4 and 5 are of the same type as the pumping light sources 2 and 3 but will differ from the pumping light sources 2 and 3 in output wavelength. This indicates that, even if the pumping light sources 4 and 5 are of the same type as the pumping light sources 2 and 3, their wavelengths do not necessarily match because of variations in manufacturing process. The polarization-coupled pumping light from the pumping light sources 2 and 3 and the polarization-coupled pumping light from the pumping light sources 4 and 5 are coupled by the WDM coupler 7. The light output from the WDM coupler is sent to an optical amplification medium, for example, an erbium doped fiber (EDF), for use as pumping light for amplifying light signals. The WDM coupler is so named because it couples wavelength-multiplexed signal light and pumping light of a specific wavelength, but in practice it can be an ordinary optical coupler.
This arrangement is used when only one pumping light source cannot provide a sufficient amplification action to the optical amplification medium and intended to obtain pumping light with a larger power through the use of two or more pumping light sources.
The pumping light source unit of FIG. 1B comprises one pumping light source driver 1, one pumping light source 8, and an optical splitter 9 for separating pumping light from the pumping light source 8. This arrangement is used when the pumping light source has a power large enough to supply two or more optical amplification media (not shown). This arrangement allows the two or more optical amplification media to be operated equally with one pumping light source having a single wavelength and a single polarized wave.
The pumping light source unit of FIG. 1C comprises two or more pumping light source drivers 1, an equal number of pumping light sources 8, and an optical coupler/splitter 10 that couples and separates pumping beams of light from the pumping light sources. This arrangement is intended to supply two or more optical amplification media with pumping light from a single pumping light source unit, but it has two or more pumping light sources 8 to provide pumping light with higher power because a single pumping light source alone cannot give all of the optical amplifiers a sufficient amplifying action. As described above, however, there are variations in wavelength between two or more pumping light sources 8. Thus, the use of each of the pumping light sources 8 for a respective one of the optical amplification media will cause their respective amplification actions to vary. For this reason, this pumping light source unit is arranged such that beams of pumping light from the pumping light sources 8 are first coupled to produce a single beam of light and then the single beam of light is separated to thereby supply each of the optical amplification media with pumping light of the same property.
Hereinafter, what includes a pumping light source unit, an optical amplification medium or media, and other circuits including a control circuit shall be called an optical amplifier.
In an optical amplifier, automatic gain control (AGC) or automatic level control (ALC) is sometimes performed to control the supply amount of pumping light to an optical amplification medium. In conventional optical amplifiers, the amount of output light of a pumping light source is varied to vary the supply amount of pumping light to the optical amplification medium by changing a drive current to the pumping light source.
In the WDM transmission system, signal light beams of different wavelengths are collectively amplified by an optical amplifier. After the start of system implementation, the WDM transmission system is sometimes modified to increase the signal multiplexing degree (i.e., the system is upgraded). When the multiplexing degree is increased, the optical amplifier requires more pumping power in order to increase the supply amount of pumping light to the optical amplification media.
In the pumping light source units shown in FIGS. 1B and 1C, the ratio of the output power of each pumping light is fixed to the ratio of the splitting by the splitter 9 or the optical coupler/splitter 10, and the amount of pumping light at each output port cannot be varied arbitrarily.
The optical amplifier contains components that are associated with a pumping light source to radiate heat generated by it. These components are relatively large among components composing the optical amplifier, preventing the optical amplifier from becoming reduced in size. When LDs as pumping light sources that generate heat and driver circuits therefor are placed close together within the optical amplifier, it will create an excessive rise in temperature, reducing the performance and reliability of the optical amplifier.
Heretofore, even if the WDM transmission system is upgraded, pumping light can only be output up to the allowable maximum output of a pumping light source installed in an optical amplifier at the beginning of system implementation. As an example, assume that, in a 16-channel WDM transmission system, only four channels are employed at the beginning of system implementation. In this case, the optical amplifier used is naturally equipped with pumping light sources necessary to accommodate 16 channels, which increases the initial investment at the time of system installation.
In an optical communications device equipped with an optical amplifier of the built-in pumping light source type, heat generated in the device is difficult to radiate and it is therefore necessary to cool the device with a fan, dissipating extra power.
When pumping light of a narrow spectrum width emitted by an LD as a pumping light source reflects from optical parts composing an optical amplifier or a fiber junction back to the pumping LD, the operation of the LD becomes unstable, which makes the operation of the optical amplifier unstable. To avoid this problem, conventionally the optical amplifier has an optoisolater built in on the output side of the pumping LD. This arrangement requires more optical parts.
In the most used type of an optical amplifier, the amplification characteristic of an amplification medium is wavelength-dependent, and pumping LDs have variations in wavelength due to variations in the manufacturing process. For this reason, the optical amplifier has variations in amplification characteristic due to variations in wavelength.
In an optical amplifier in which a pumping light source unit and an optical amplification medium are coupled together by means of a connector, it is necessary to sound an alarm in the case that the connector has come off, because pumping light leaking out through the connector is very dangerous for persons at work.
It is an object of the present invention to provide an optical amplifier which has a function of changing the supply amount of pumping light to an amplification medium, and is small in size and little affected by heat generated by a pumping light source.
According to a first aspect of the present invention, there is provided an optical amplifier for amplifying incoming signal light in response to pumping light applied thereto, characterized by including a variable attenuator for varying the input level of the pumping light applied to the optical amplifier to thereby tune the amplification characteristic of the optical amplifier.
In the optical amplifier of the present invention, two or more optical amplification units that contain amplification media are assembled into one package.
The optical communications device of the present invention comprises a pumping light source unit having at least one pumping light source for generating pumping light and a unit for separating or coupling pumping light from the at least one pumping light source, and an optical amplification unit for amplifying incoming signal light in response to application thereto of pumping light from the pumping light source unit and is characterized in that the pumping light source unit is placed in a location in the optical communications device where heat radiation conditions are good.
According to a second aspect of the present invention, there is provided an optical amplifier for amplifying incoming signal light in response to application thereto of pumping light from a pumping light source unit having a pumping light source for generating a pumping light beam and an optical coupler unit for coupling multiple pumping light beams, characterized in that the pumping light source unit includes a polarization plane rotating unit for rotating the plane of polarization of output pumping light from the optical coupler unit through a first angle of rotation for transmission and rotating the plane of polarization of return light, resulting from the output pumping light being reflected from a connector connecting the pumping light source unit and other components of the optical amplifier back to the pumping light source unit, through a second angle of rotation, thereby inputting to the pumping light source return light, different in wavelength from the pumping light source generated by the pumping light source.
According to a third aspect of the present invention, there is provided an optical amplifier for amplifying incoming signal light in response to application thereto of pumping light from a pumping light source unit having multiple pumping light sources each generating a pumping light beam and an optical coupler/splitter unit for coupling multiple pumping light beams and splitting into individual light beams, characterized in that the pumping light source unit includes a polarization plane rotating unit for rotating the plane of polarization of output pumping light from the optical coupler unit through a first angle of rotation for transmission and rotating the plane of polarization of return light, resulting from the output pumping light being reflected from a connector connecting the pumping light source unit and other components of the optical amplifier together back to the pumping light source unit, through a second angle of rotation, thereby inputting to the pumping light source return light different in wavelength from the pumping light source generated by the pumping light source.
According to a fourth aspect of the present invention, there is provided an optical amplifier in which a pumping light source unit having a pumping light source for generating pumping light and an optical amplification unit having an amplification medium for amplifying incoming signal light in response to application of the pumping light thereto are connected together by means of a connector that allows the pumping light to be transmitted to the optical amplification unit, characterized in that the optical amplification unit includes a unit for determining whether or not the connection between the pumping light source unit and the optical amplification unit is established by means of the connector on the basis of the output level of the pumping light from the pumping light source unit.
According to a fifth aspect of the present invention, there is provided an optical amplifier in which a pumping light source unit having a pumping light source for generating pumping light and an optical amplification unit having an amplification medium for amplifying incoming signal light in response to application of the pumping light thereto are connected together by means of a connector that allows the pumping light to be transmitted to the optical amplification unit, characterized in that the pumping light source unit includes a unit for determining whether or not the connection between the pumping light source unit and the optical amplification unit is established by means of the connector on the basis of the level of return light reflected from the connector.
According to a sixth aspect of the present invention, there is provided an optical amplifier in which a pumping light source unit having a pumping light source for generating pumping light and an optical amplification unit having an amplification medium for amplifying incoming signal light in response to application of the pumping light thereto are connected together by means of a connector that allows the pumping light to be transmitted to the optical amplification unit, characterized by comprising a unit for determining whether or not the connection between the pumping light source unit and the optical amplification unit is established by means of the connector.
An optical amplification unit of the present invention has an amplification medium for amplifying incoming signal light in response to application thereto of pumping light from a separate pumping light source unit, the optical amplification unit and the pumping light source unit being connected by a connector to form an optical amplifier, and is characterized by the provision of a variable attenuator for adjusting the level of pumping light input to the amplification medium.
A pumping light source unit of the present invention has a pumping light source for generating pumping light to be output to a separate optical amplification unit, the pumping light source unit and the optical amplification unit being connected by a connector to form an optical amplifier, and is characterized by the provision of a variable attenuator for adjusting the level of pumping light to be output to the optical amplification unit.
According to the present invention, the pumping light, which, in the conventional system, has its output level maintained constant or adjusted by controlling the pumping light source itself, can be level-adjusted easily by the use of the variable attenuator, which allows pumping light with a suitable intensity to be supplied to the amplification medium.
In addition, since the pumping light source is separated from the optical amplification unit, two or more optical amplification units can be grouped into one package, ensuring compactness of the optical amplifier.
Being separated from the optical amplification unit, the pumping light source can be placed in a location where heat radiating conditions are good and can suppress the effect of heat on the light amplification unit.
By the provision of the unit for rotating the plane of polarization of pumping light output from the pumping light source through a predetermined angle in the pumping light source unit, the operational instability of the light source due to return light can be eliminated.
According to the present invention, the optical amplifier is separated into the pumping light source unit containing the pumping light source and the optical amplification unit containing the amplification medium and a connector is therefore required to couple these units together. The connector may come off while a person is working. Not only has the pumping light high power, but it is converged by an optical fiber. In the event that the connector has come off, therefore, the person at work may be exposed to danger. To avoid such danger, the optical amplifier of the present invention is provided with means for detecting whether the connector is off or not.